1. Field of the Invention
This invention relates to a newly developed mechanical plating material and a mechanical plating method that uses such a plating material, thus making a zinc alloy film on the surface of a metallic substrate, mainly steel.
2. Prior Art
The conventional mechanical plating method, which forms a zinc film on the surface of metallic substrates or works by ejecting an ejection material which is zinc powder, is known as the galvanizing method. The ejection powders proposed in the prior art-are classified into two groups. One is the mixture of steel shot and zinc powder (or zinc particle) as disclosed in British Patent No. 1041620, and the other is zinc alloy particles each having an iron nucleus as disclosed in the Japanese Patent Application Publication (Kokoku) No. 59-9312. The ejection powder having iron nuclei is superior, because all the ejection powders can participate to form the zinc alloy film. At present, this method is most commonly used.
There are two methods of manufacturing zinc alloy powders having iron nuclei. One is a melting-zinc method in which nuclei of iron alloy (which include not only pure iron but also an iron alloy that contains Ni, Cr, Mn, etc.) is dipped into a zinc alloy melt (which includes not only pure zinc but also a zinc alloy that contains Cu and Al) to form a zinc alloy layer around the iron alloy nucleus. The other method is a zinc penetration method in which iron alloy particles are treated with zinc alloy vapor. In either method, the zinc alloy layer formed around the iron alloy nucleus is composed of several different phases. At the inner most there is the .alpha.(alpha) phase, which is a solid solution of iron alloy and zinc alloy, then there are the .gamma.(gamma) phase, the .delta.(delta) phase and the .zeta.(zeta) phase which are metallic compounds of iron alloy and zinc alloy, and then there is the .eta.(eta) phase that is a zinc alloy phase. The .eta.(eta) phase is located at the outer most of the particle and is very soft, having Vicker's hardness (Hv) ranging from 30 to 60.
When the ejection powder thus obtained is ejected onto metallic works, a zinc alloy film is formed on the surface. It is believed that the film is formed because a part of the zinc alloy layer of the ejection powder is destroyed when the ejection powder collides against the metallic works and stay on the surface of the metallic works. Accordingly, the easily destructive nature of the zinc alloy layer of the ejection powder is a significant factor for a zinc alloy film to form on the surface of metallic works. On the other hand, if the zinc alloy phase is too soft, it is difficult to destroy, and the zinc alloy film does not form on the metallic works.